Solids Circulation Rate Research Articles

Oily wastewater treatment using a CNT sorbent in a three-phase circulating fluidized bed

Fine emulsified heavy-oil droplets in wastewater can be easily dispersed in aquatic system, making it challenging to remove them using sorbents. Carbon nanotube (CNT) sorbents were prepared using a m-cresol solvent to remove emulsified heavy oil in wastewater. The bead-shaped and density-controlled sorbents (diameter: 5.22 mm; density: 1169 kg/m3) facilitated efficient contact with emulsified oil and were suitable for use in a three-phase circulating fluidized bed (TPCFB). The sorbents feature a sorption capacity of 2.4 g oil/g sorbent which remained consistent for at least 10 cycles. The hydrodynamic characteristics of the TPCFB reactor (120 × 80 mm; height: 800 mm) were determined and it was found that the circulation of both liquid and solid within the TPCFB was driven by the density difference between the riser and downcomer (Δρ). The solid holdups in the downcomer (εs,d) were almost constant with gas velocity and were proportional to the amount of sorbent inventory. The optimal region for emulsified oil removal was determined to be the downcomer, where turbulent flow was generated that facilitated close contact between the particles and the liquid. The solid circulation rate (Gs) increased to a constant value with Δρ, with the maximum Gs being proportional to the sorbent inventory. The optimal conditions for wastewater treatment were determined to be a maximum εs,d of 0.03 with a Gs value above 10.0 kg/m2s and a sorbent inventory of 90 g. The oil removal rate reached a maximum of 94.8 % at Gs = 12.2 kg/m2s. Used CNT sorbent exhibited unique oil sorption characteristics of CNTs, featuring an even distribution of sulfur on the surface and a weight loss arising from loss of oil at 350–500°C.

Hydrodynamics and operation of a multistage fluidized bed with particles of super-high density

High-density particles are commonly used in fluidized bed reactors in uranium recycling processes of nuclear industry, where high fluidization quality (i.e. low gas-solids contacting efficiency) is difficult to achieve, resulting in various problems such as reduced conversion, low product selectivity, stronger equipment vibration and bad operating stability, etc. The objective of this study was to validate the feasibility and advantages of a multistage fluidized bed in processing a super-high-density powder. Wolfram carbide powder of true density of 15630 kg/m3 was used in a cold-model counter-current three-stage fluidized bed with downcomers. Stability of operation, operating range and hydrodynamics were systematically studied. The fluidization quality and operating stability were also compared with a single-stage conical fluidized bed of similar diameter and same solids inventories. The experimental results demonstrated that this high-density wolfram carbide powder could be fluidized and circulated continuously at this multistage fluidized bed, but lower fluidization quality was observed and the stable operating range is narrow. Narrower stable operating range was observed at higher solids circulation rates. At corresponding operating conditions, the fluidization quality of multistage fluidized bed is significantly better than that of the conical fluidized bed with a similar solids inventory.

Computational analysis of the effect of bed particles in a dual chamber internally circulating fluidised bed

ABSTRACT Internal circulation of particles in a dual-chamber fluidised bed can be developed by applying air of unequal velocity at the bottom of each chamber. This internal circulation results in an energy transfer between the chambers which is desirable in several industrial applications. ANSYS-Fluent software is used in this paper to simulate the influence of bed particles on the flow behaviour of gas and solids in a dual-chamber fluidised bed. The results of the simulation are expressed as solids circulation rate (Gs ) and discharge coefficient (Cs ). Simulations show that Cs at the slot ranged from 0.50 to 0.60 whereas Gs in the reactor is significantly affected by varying the bed particles. The pressure difference (due to the difference in density) between the chambers develops the internal circulation of particles in the dual-chamber internally circulating fluidised bed. It was observed that the denser the bed material, the higher the internal circulation is along with the pressure difference near the slot, and vice versa. Therefore, it can be concluded that the ICFB’s performance does depend on the bed particle type.

Research on the generalization issue of the heterogeneous QC-EMMS drag model for gas-solid fluidization

In the circulating fluidized bed (CFB), the non-uniform mesoscale structure formed by the particle clustering effect causes a substantial decrease in the gas-solid drag. Since the degree of particle clustering varies with operating conditions, an accurate drag model needs to be universal in different heterogeneous flow conditions. In this study, the relationships between the Ψ factor in clusters' solid holdup model that characterizes the flow non-uniformity and the operating parameters of CFB (including slip velocity Reynolds number, Re*, bed-averaged solid volume fraction, εs,bed, and solid mass circulation rate, Gs) are established. It improves the adaptability of the QC-EMMS drag model under different working conditions. In previous research, we established two types of models that related Ψ factor to Re* and εs,bed respectively. However, there exists a problem of high dispersion of points, indicating that the selected parameters cannot fully describe the flow non-uniformity. Therefore, Gs is reintroduced to modify the two types of models. The results show that the prediction accuracy of the modified models is improved and the relative error is <10%, indicating that the non-uniform factor Ψ has a strong correlation with Gs. In addition, the quantitative relation between Re*, εs,bed, and Gs is derived from modified models, and the trend of relation is highly consistent with the fluidization diagram proposed by Yerushalmi J., which verifies the accuracy of modified models. Finally, numerical simulation of typical CFB cases proves the adaptability of the modified models in wide operating conditions of fluidization.

Numerical Simulation Study on the Gas–Solid Flow Characteristics of a Large-Scale Dual Fluidized Bed Reactor: Verification and Extension

Dual fluidized bed (DFB) reactor systems are widely used in gas–solid two-phase flow applications, whose gas–solid flow characteristics have a significant effect on the performance of many kinds of technologies. A numerical simulation model was established on the basis of a large-scale DFB reactor with a maximum height of 21.6 m, and numerical simulations focused on gas–solid flow characteristics were carried out. The effects of the superficial gas velocity of both beds and the static bed height and particle size on the distribution of the pressure and solid suspension density and the solid circulation rate were studied. The simulation results were in good agreement with the experimental data. With the strong support of the experimental data, the gas–solid flow characteristics of large-scale DFB reactors were innovatively evaluated in this numerical simulation study, which effectively makes up for the shortcomings of the current research. The results showed that the superficial gas velocity of both beds and the static bed height have different degrees of influence on the gas–solid flow characteristics. Specifically, for 282 μm particles, when the superficial gas velocity of both beds and the static bed height were 4.5 m/s, 2.5 m/s, and 0.65 m, respectively, under typical working conditions, the bottom pressure of the two furnaces was 3412.42 Pa and 2812.86 Pa, respectively, and the solid suspension density was 409.44 kg/m3 and 427.89 kg/m3, respectively. Based on the simulation results, the empirical formulas of the solid circulation rate were fitted according to different particle sizes. Under similar conditions, the solid circulation rates of particles with a particle size of 100 μm, 282 μm, 641 μm, and 1000 μm were 2.84–13.28, 0.73–4.91, 0.024–0.216, and 0.0026–0.0095 kg/(m2s), respectively. It can be found that the influence of the particle size on the solid circulation rate is the most significant among all parameters.

Physical modelling and hydrodynamic characterization of an internally circulating fluidized bed for biomass fast pyrolysis

This study investigates the hydrodynamics of the cold flow model (CFM) of an internally circulating fluidized bed (ICFB) finalized to biomass fast pyrolysis. The ICFB, basically consisting of a bubbling fluidized bed interconnected with a moving bed, is designed aiming at high fuel particles heating rates, efficient solids mixing and low gas residence times. Experimental techniques such as basket collection, solids and gas tracing are adopted to determine solids circulation rates, gas bypassing and particles residence times, consequently a compartment model of the CFM is proposed. The influence of operating conditions such as bed inventory and superficial gas velocity is investigated together with the effect of the opening ratio between the two interconnected beds. Results highlight that different operation regimes can occur and an optimization of operating and design parameters allows high solids circulation rates, low gas bypass and residence times lower than 1 s, as pursued in fast pyrolysis.

Simulation of the L-valve in the circulating fluidized bed with a coarse-grained discrete particle method

Stable and controllable solid flow is essential in circulating fluidized bed (CFB) systems. The L-valve is a typical non-mechanical valve that can provide flexible solid feeding. The investigation of the solid circulation rate and the hydrodynamic characteristics of the L-valve is crucial to its design and operation. The gas-solid flow in the L-valve of a full-loop CFB is studied with the coarse-grained discrete particle method (EMMS-DPM). Good agreements on the solid circulation rate and the pressure drop through the L-valve are achieved between the simulated and experimental data. The solid circulation rate increases linearly with the aeration velocity until the stable particle circulation of the CFB is destroyed. The flow patterns in the horizontal section of L-valve are gas-solid slug flow above the stationary solid layer and the moving solid layer, respectively. The effects of L-valve geometric parameters on the solid flow characteristics are also investigated. The results indicate that reducing the diameter and length of the horizontal section of L-valve can improve the solid transport efficiency, especially at low aeration velocity. Besides, the solid conveying capacity and flow stability are improved when the sharp bend of L-valve is modified to be a gradual bend.

Gaussian Process Regression-Based Control of Solids Circulation Rate in Dual Fluidized Bed Gasification

Gaussian Process Regression-Based Control of Solids Circulation Rate in Dual Fluidized Bed Gasification

Oxygen Carrier Circulation Rate for Novel Cold Flow Chemical Looping Reactors

To achieve net-zero emissions by the year 2050, carbon capture, utilization, and storage technologies must be implemented to decarbonize sectors with hard-to-abate emissions. Pressurized chemical looping (PCL) with a novel reactor design called a plug flow with internal recirculation (PFIR) fluidized bed is proposed as an attractive carbon capture technology to decarbonize small- and medium-scale emitters. The objective of this work is to examine the solid circulation rate between redox reactors in a cold flow chemical looping facility using an energy balance approach. The effects of static bed height, weir opening height, purge configuration, and gas flow rate on solid circulation rate were investigated. It was determined that parameters that greatly affected the total gas momentum, such as the fluidization ratio or number of purge rows, tended to also have a large effect on solid circulation rate. Parameters that had a small effect on total gas momentum, such as bed height, did not have a measurable effect on solid circulation rate. It was noted that parameters that posed a restriction to solids flow, such as a vertical purge jet or the weir itself, decreased the solid circulation rate compared to similar tests without restrictions.

A method for measuring the solid circulation rates of CFB boilers based on the particle flow-around principle

The solid circulation rate (Gs) is a key parameter for the design and operation of a circulating fluidized bed (CFB) industrial equipment, while the detection and measurement of the Gs value in an actual industrial CFB equipment remains difficult. To fill this gap, a method for the Gs measurement based on the flow-around principle was proposed in this study. The mathematical model for the signal transformation was established along the path “detected strain value – impact force – particle velocity – Gs value”, and the corresponding measurement device was also developed. The performance of the Gs measurement device was quantitatively validated in cold-state experiments and semi-quantitatively verified in an industrial trial in a CFB boiler. The results of a theoretical analysis indicate that the detected strain linearly increased with the impact force received by the intrusive spherical obstacle, and the slope coefficient was determined as per the experiment. The correlation of the impact force to the particle flow velocity was proposed and fitted using the experimental data. The computational particle fluid dynamics method was applied to simulate the particle flow field in the standpipe of the CFB boiler to determine the installation location of the Gs measurement device. The lab-scale cold-state experiments showed that the relative standard uncertainty of the Gs measurement using the device built in the present study was less than 2.0 %. This device was then applied in an industrial trial in a 116 MWth CFB boiler and the results proved the feasibility and applicability of the proposed Gs measurement method.

Experimental Research on the Gas-Solid Flow Characteristics in Large-Scale Dual Fluidized Bed Reactor

A dual fluidized bed (DFB) reactor is the main operating system of various energy-efficient and clean utilization technologies. The gas-solid flow characteristics of the DFB reactor greatly affect the efficiency of various technologies. A large-scale DFB reactor with a maximum height of 21.6 m was built and relevant cold mode tests were carried out in this study. The effects of the superficial gas velocity of both beds, static bed height and particle size on the distribution of both pressure and solid suspension density, solid circulation rate, solid inventory distribution ratio and other characteristics were studied. For 282 μm-particles, the solid suspension density in the dense phase zone of the two beds was 100–400 and 400–800 kg/m3, respectively, when the static bed height was 0.65 m; the solid circulation rate was about 0.87–1.75, 1.04–3.04 and 1.13–3.69 kg/(m2s) when the static bed height was 0.65, 0.95 and 1.25 m, respectively. The solid circulation rate was positively correlated with the static bed height and the superficial gas velocity of both beds, yet negatively correlated with the particle size. Additionally, the empirical equation of solid circulation rate and the empirical equation of solid inventory distribution ratio were proposed, respectively. The material control method of the DFB reactor is put forward.

Multibubble Behavior and Solid Circulation Rate in a Gas–Liquid–Solid Circulating Microfluidized Bed

Multibubble Behavior and Solid Circulation Rate in a Gas–Liquid–Solid Circulating Microfluidized Bed

Catalyst-enhanced autothermal chemical looping reforming: Intrinsic SMR kinetics and numerical simulation

In this work, we propose a catalyst-enhanced autothermal chemical looping reforming (CE-aCLR) to address the environmental concerns associated with toxic Ni-based oxygen carrier (OC) and the low fuel conversion related to the low reactivity of Fe-based OC. In this process, a non-toxic noble-metal-based steam methane reforming (SMR) catalyst is employed for methane conversion, while a Fe-based OC is employed for oxygen transfer between two reactors. Firstly, a commercial Rh-based SMR catalyst is studied in a micro-fixed bed reactor and its intrinsic reaction kinetics are determined. Then, a 1-D CE-aCLR model is developed based on a verified multi-scale aCLR model to facilitate analyzing commercial-scale feasibility. This model accounts for the essentials of the reactor hydrodynamics, mass and heat transfer, detailed reaction kinetics, and the influence of the solids residence time distribution in the fuel reactor. The main dimensions of a commercial CE-aCLR unit are determined and this unit is simulated using the developed model. The results demonstrate the feasibility of the novel, environmentally friendly process that enables high methane conversion using Fe-based OC within realistically sized reactors and an acceptable solid circulation rate. Moreover, sensitivity study shows that the CE-aCLR allows using OCs with low activity (potentially low cost) while maintaining high CH4 conversion.

An investigation of the aeration proportion effect at the supply and recycle chambers on the optimal operation point of the U-type loop seal and the maximum solid circulation rate in a fluidized bed reactor

A loop seal is used to control a solid circulation rate and loop seal aeration ratios at the supply and recycle chambers are another factor that affects its efficiency and circulating behaviors of solid particles in a fluidized bed reactor. This research aimed to find the proper aeration proportion and operation point of the loop seal that affected the highest solid circulation rate. A riser used in the experiment had a cross-sectional area of 100 × 100 mm2 and 4500 mm height above a distributor plate. A U-type loop seal was constructed by a supply chamber and a recycle chamber with the same cross-sectional area. A horizontal passage of 50 mm length was connected with both chambers. The sand was a bed particle with an average diameter of 320 μm. The loop seal aerations were fed at the bottom of the supply and recycle chambers. It was found that the loop seal could not operate with supply-aeration flow rate percentages (PQS) of 0% and 100%. The PQS of about 73% gave the maximum solid circulation rate and was an optimal operation point at which air supplied at the supply chamber started bypassing into the recycle chamber.

Experimental study of solid circulation rate and pressure distribution characteristics in a cold fluidized bed with an external heat exchanger in an municipal solid waste incinerator

AbstractThe external heat exchanger (EHE) can solve the problem of the insufficient heating surface due to the large‐scale circulating fluidized bed (CFB) boiler effectively; furnace temperature and bed temperature can be regulated quickly and accurately with the adjustment of the solids circulation rate (SCR). The ash differs from coal‐fired boilers in particle size, density, and other physical parameters, so the circulation and heat transfer characteristics are not identical. The results of SCR and pressure distribution in a cold CFB experimental system with an EHE in which the height of the partition (HP) could be adjusted were analyzed. Experiments studied the effect of fluidized air velocity in the riser (Ug), HP of the EHE, and the pressure of the supply air chamber (PSA) in the EHE on SCR using quartz sand with an average size of 0.239 mm. It was noted that SCR increased with Ug and PSA in the range of 7.01–28.37 g/s with the increase in Ug, and the maximum value of SCR was obtained when PSA was 750 Pa and the HP was 80 mm. The experiments also noted that the increase in SCR significantly affected the pressure distribution in the region of high solids content (EHE and the region above 600 mm in the riser). A reference to the above characteristics of SCR and pressure distribution variations can be considered for operating commercial CFB waste incinerators.

Experimental investigation of circulating fluidized bed combustion of dry powders of coal slime

Effective combustion of coal slime in circulating fluidized bed (CFB) boilers is essential for the sustainable development of clean coal technology. To overcome issues related to burning wet coal slime in CFBs, we propose to dry coal slime in a low-temperature drying system and burn the dried fine powders in a CFB with a highly efficient separator. This study reports a series of experiments conducted in a 4MWth pilot-scale CFB combustion test facility to assess the feasibility. The results demonstrate that the dried slime particles, which have a particle size distribution similar to that of circulating ash particles, can be burned stably, efficiently, and cleanly. The combustion of fuels with size distributions similar to that of circulating ash particles increases solids circulation rates, betters solids concentration distribution, enhances heat transfer combustion and improves operation and SO2 and NOx controls. Therefore, the study presents a potential opportunity to advance CFB combustion technology.

Numerical study of the biomass gasification process in an industrial-scale dual fluidized bed gasifier with 8MWth input

In the present numerical study, biomass steam gasification in industrial-scale dual fluidized bed gasifier with 8MWth input is numerically simulated via the multiphase particle-in-cell (MP-PIC) method, where the gas turbulence is controlled via large eddy simulation (LES), and the solid phase is tracked under the Lagrangian framework. This model has been well validated with experimental measurement. The gas and solid fluxes, gas distribution, gas and solid thermal properties together with the effect of solid inventory and steam-to-biomass ratio on the system performance are discussed in detail. Results show that the asymmetrical structure of the gasifier leads to a nonuniform distribution of gas phase. Product gases released in the gasifier have similar distributions. The vertical gas flux is much higher than those along radial directions, due to the fluidization characteristics. A decrease of the vertical gas temperature in the combustor and an increase of that in the gasifier are observed due to the improvement of solid circulation rate and heat transfer raised by increasing the solid inventory. Enlarging the solid inventory can increase the particle mean temperature at gasifier outlet while decreases that at gasifier inlet, and decrease the sand temperature and heat transfer coefficient along the combustor height. Moreover, the impact of inventory height and S/B on gas thermal properties in both reactors is discussed.

Assessment of experimental methods for measurements of the horizontal flow of fluidized solids under bubbling conditions

Dual fluidized bed systems are indispensable for future energy systems that require solids cycling between different atmospheres. However, controlling the residence time of solids in the reactor, which is crucial for controlling the heat and mass transfer of the fuel, is a significant challenge. This study investigates four experimental techniques to quantify the horizontal flow of solids fluidized under bubbling conditions: integral mass accumulation; differential mass accumulation; thermal tracing; and magnetic solids tracing. Integral mass accumulation entails collecting bed material using a defluidized box within a given time period. Differential mass accumulation measures the material accumulation rate in a section of the bed that is monitored using pressure measurements. Thermal tracing calculates the solids flow rate by solving the heat balance to match the temperature field captured by a thermographic camera. Magnetic solids tracing involves injecting a batch of magnetic tracer solids into the reactor and then measuring the residence time distribution using impedance coils.The experiments were conducted under down-scaled conditions that resemble large-scale operations with a length scaling factor of 0.12. For this study, three operational parameters were varied: the fixed bed height; the volumetric flow rate of the conveying air; and the fluidization velocity in the bed. The horizontal solids circulation rates achieved ranged from 1.7×10−4 to 10 kg/m·s, corresponding to 1.2×10−3 to 70 kg/m·s on a hot up-scaled basis, which is a relevant range to indirect biomass gasification in an industrial setting.The three selected operational parameters led to increases in the horizontal solids flow. While all four methods replicated the trends, quantitative variations in the measured circulation rates occurred due to the inherent characteristics of the methods. High circulation rates resulted in a continuous decrease in the solids inventory, leading to an underestimation of the circulation rate when using the integral mass accumulation method. The accuracy of the differential mass accumulation method relied on transient pressure measurements, which were less-effective at low solids flow rates. Conversely, the accumulation time required for pressure measurements was reduced at high circulation rates, resulting in uncertainties in the analysis. The accuracy of the thermal tracing method decreased drastically with higher solids circulation, resulting in an overestimation of the circulation rate. Moreover, low circulation rates adversely affected the accuracy of the magnetic solids tracing by producing barely discernible tracer concentration gradients.

Directly irradiated fluidized bed autothermal reactor (DIFBAR): Hydrodynamics, thermal behaviour and preliminary reactive tests

The advancements of the Concentrating Solar Thermal (CST) technologies in the heat and power sector are pushing researchers to find new ways to exploit solar energy. Solar-driven thermochemical processes can open new scenarios and set a milestone towards a greener industry. This paper presents the development of a Directly Irradiated Fluidized Bed Autothermal Reactor (DIFBAR), that exploits fluidization technology and the principle of an autothermal reactor to carry out solar chemical processes with high efficiency. The viability of recovering the sensible energy of the solid products to preheat the reactants is studied in an experimental prototype that couples a cavity receiver/reactor and a countercurrent double-pipe heat exchanger. The prototype is operated as a circulating fluidized bed: the inner tube of the heat exchanger is a fluidized bed riser, the receiver works as a gas–solid separator, the outer tube (annulus) of the heat exchanger is an overflow standpipe and a buffer tank (reservoir) connects the annulus to the riser, closing the loop. The reservoir can also be operated as a fluidized bed reactor like in a dual-fluidized bed system. The first experimental results are reported using a Geldart B sand as bed inventory. A hydrodynamic study has been carried out to verify proper control of the system. Solid circulation rates have been determined and match the design target of 1.4 g/s. Pressure measurements have been used to control bed levels and the flow of the sand through the standpipe. The effects of gas velocities and outlet pressure drops are reported. Internal gas flow patterns have been determined by a gas tracing technique. Undesired gas by-passing streams are very small and can be zeroed by regulating the operating conditions. High temperature experiments have been conducted with a high-flux solar simulator under inert and reactive conditions, to prove the operating principle of the reactor. Steady state temperature profiles have been analyzed to assess the performance of the heat exchanger: the heat transfer coefficient ranges between 340 and 490 W/(m2 K). The operability of a solar-driven chemical process has been proved by calcining a batch of magnesium carbonate (MgCO3) particles, added to the inventory.

Thermal decomposition mechanism analysis of circulating solids sampled from Ende pulverized‐coal gasifier

AbstractEnde pulverized‐coal gasifier (EPCG) is a type of circulating fluidized bed gasifier, in which the circulating solids are resent into the furnace through a return conduit for regasification. Since the solid circulation in the return conduit mainly relies on gravity self‐balancing, it is easy to cause low and unstable solid circulation rate and increase unburned combustible of fly ash. To solve these problems, a circulation enhancement technique using an ejector to provide a stable pressure barrier to promote solid circulation was proposed. To verify its feasibility, considering that the circulating solid‐CO2 reaction is a rate‐controlling step in the gasification process, it is of great significance to explore the reactivity characteristics of the circulating solids under CO2 atmosphere. In this work, thermal decomposition characteristics of Hailar circulating solids (HLE‐CS), Hailar char (HLE‐C), and Shenhua circulating solids (SH‐CS) were investigated by non‐isothermal thermogravimetric analysis to compare the gasification reaction characteristics of circulating solids and char. The HLE‐CS and SH‐CS used in the experiments were taken from the return conduit of 45,000 and 20,000 Nm3/h EPCGs, respectively, while HLE‐C was made from the raw coal of 45,000 Nm3/h EPCG. The effects of different samples under the same thermal decomposition temperature range were investigated. The kinetic parameters were determined by the distributed activation energy model. The results indicated that remarkable peaks were found in the relationship between temperature (T) and activation energy (E) for HLE‐CS and SH‐CS, but there was no significant peak for HLE‐C, and kinetic compensation effects also existed between ln(k0) and E for different samples. The circulating solids of HLE‐CS and SH‐CS were more easily gasified than that of HLE‐C in the lower temperature ambience. Moreover, HLE‐CS and SH‐CS have obvious pyrolysis and gasification stages, which can be completely introduced into EPCG for regasification, and increasing the circulating solid rate and temperature within a certain range is beneficial to enhance the gasification rate.